Malcolm Connolly

Gigahertz quantized charge pumping in graphene quantum dots

Single-electron pumps are set to revolutionize electrical metrology by enabling the ampere to be redefined in terms of the elementary charge of an electron. Pumps based on lithographically fixed tunnel barriers in mesoscopic metallic systems and normal/superconducting hybrid turnstiles can reach very small error rates, but only at megahertz pumping speeds that correspond to small currents of the order of picoamperes. Tunable barrier pumps in semiconductor structures are operated at gigahertz frequencies, but the theoretical treatment of the error rate is more complex and only approximate predictions are available. Here, we present a monolithic, fixed-barrier single-electron pump made entirely from graphene that performs at frequencies up to several gigahertz. Combined with the record-high accuracy of the quantum Hall effect and proximity-induced Josephson junctions, quantized-current generation brings an all-graphene closure of the quantum metrological triangle within reach. Envisaged applications for graphene charge pumps outside quantum metrology include single-photon generation via electron-hole recombination in electrostatically doped bilayer graphene reservoirs, single Dirac fermion emission in relativistic electron quantum optics and read-out of spin-based graphene qubits in quantum information processing.

Scanning gate microscopy of current-annealed single layer graphene.

We have used scanning gate microscopy to explore the local conductivity of a current-annealed graphene flake. A map of the local neutrality point (NP) after annealing at low current density exhibits micron-sized inhomogeneities. Broadening of the local e-h transition is also correlated with the inhomogeneity of the NP. Annealing at higher current density reduces the NP inhomogeneity, but we still observe some asymmetry in the e-h conduction. We attribute this to a hole-doped domain close to one of the metal contacts combined with underlying striations in the local NP.

Atomic force microscope nanolithography of graphene: Cuts, pseudocuts, and tip current measurements

We investigate atomic force microscope nanolithography of single and bilayer graphene. In situ tip current measurements show that cutting of graphene is not current driven. Using a combination of transport measurements and scanning electron microscopy we show that while indentations accompanied by tip current appear in the graphene lattice for a range of tip voltages, real cuts are characterized by a strong reduction in the tip current above a threshold voltage. The reliability and flexibility of the technique is demonstrated by the fabrication, measurement, modification, and remeasurement of graphene nanodevices with resolution down to 15 nm.

Quantum Hall Effect and Quantum Point Contact in Bilayer-Patched Epitaxial Graphene

We study an epitaxial graphene monolayer with bilayer inclusions via magnetotransport measurements and scanning gate microscopy at low temperatures. We find that bilayer inclusions can be metallic or insulating depending on the initial and gated carrier density. The metallic bilayers act as equipotential shorts for edge currents, while closely spaced insulating bilayers guide the flow of electrons in the monolayer constriction, which was locally gated using a scanning gate probe.

Encapsulation of graphene transistors and vertical device integration by interface engineering with atomic layer deposited oxide

We demonstrate a simple, scalable approach to achieve encapsulated graphene transistors with negligible gate hysteresis, low doping levels and enhanced mobility compared to as-fabricated devices. We engineer the interface between graphene and atomic layer deposited (ALD) Al

Imaging ballistic carrier trajectories in graphene using scanning gate microscopy

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Direct imaging of coherent quantum transport in graphene p−n−p junctions

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Evidence for formation of multi-quantum dots in hydrogenated graphene

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Numerical simulation of scanning gate spectroscopy in bilayer graphene in the Quantum Hall regime

by tailoring the growth parameters to achieve effective device encapsulation whilst enabling the passivation of charge traps in the underlying gate dielectric. We relate the passivation of charge trap states in the vicinity of the graphene to conformal growth of ALD oxide governed by

Imaging Bulk and Edge Transport near the Dirac Point in Graphene Moiré Superlattices

\textit{in situ}